Graft vs. host disease (GVHD), where donor T cells recognize and destroy alloantigen-expressing host tissues, is a major complication of allogeneic stem cell transplantation (alloSCT) limiting the use of alloSCT as a more widely applied therapy. Greater knowledge of the mechanisms driving donor T cell-mediated GVHD is needed to provide a better understanding of how to prevent or resolve GVHD. To create space for donor stem cells and prevent their rapid rejection by host immune cells, alloSCT protocols remove the recipient?s immune system and hematopoietic stem cells with conditioning regimens including chemotherapy and/or irradiation. These treatments damage the barrier tissues, including the small intestine (SI), and stimulate the release of interleukin (IL)-33 from GVHD target tissue. While GVHD is unique in that antigen is always present, the pathogenesis of the T cell mediated disease follows the paradigm of T cell activation in the secondary lymphoid organs (SLO) and migration to the barrier tissue where there is tissue damage following conditioning regimens. Interestingly, IL-33 is expressed both in the mucosal barrier tissues and SLO. Yet, it is unknown whether recipient barrier tissues, SLO, or both are the critical source of pathogenic IL-33 in GVHD. Therefore, I hypothesize that IL-33 promotes GVHD because it first amplifies early activation and differentiation in the SLO and then sustains allogeneic CD4+ T cell effectors in the GVHD target tissues. I will test this hypothesis by addressing the following Specific Aims: 1. Define how an early lack of IL-33 stimulation of alloreactive CD4+ T cells shapes activation, proliferation and differentiation in the SLO; 2. Determine if interrupting late ST2 signaling to donor CD4+ Th1 cells limits their persistence in the GVHD target tissues and stops GVHD. With successful completion of these aims, I will establish how IL-33 impacts CD4+ T cell activation in the SLO and influences CD4+ T cell survival and persistence in the SI during GVHD after alloSCT. These studies will provide new insights into how alarmins promote and sustain alloimmunity and GVHD. These studies will also elucidate if neutralizing IL-33 or interruption of ST2 signaling in the SLO or SI is a targetable therapy for preventing or resolving GVHD. Contribution to Training: This proposal describes a unique training plan combining research in the pathogenesis of GVHD and alloimmunity with bioinformatics training and professional development through presentations at conferences and manuscript preparation. This work integrates my clinical interests in hematology and oncology with renowned research programs at the University of Pittsburgh and is a strong foundation for a successful career as an academic physician scientist.